This invention relates to compositions and a method for applying coatings to metallic surfaces. These coatings are characterized by the presence of tellurium and/or selenium.
As used herein, the term "coating" refers to a material bonded to the surface of a metal which differs chemically from the metal itself. A particular example of a coating is a phosphate-based conversion coating. Such a coating is formed by chemical interaction between a phosphate-containing coating composition and the metal substrate being treated.
Conversion coatings are used to enhance the corrosion resistance of treated metal surfaces and to improve the adherence of paints and other coatings to these surfaces. As practiced in the art, conversion coatings are generally applied to metallic surfaces as iron phosphate, zinc phosphate or manganese phosphate. For example, a conversion coating may be produced by contacting a metal surface with a composition comprising a phosphate source, an acid and an accelerator. Typical accelerators used for this purpose include molybdenum, vanadium, nickel and tungsten salts. The use of tellurium and/or selenium ion sources in coating compositions is not known in the art.
Prior to application of a conversion coating, the metallic surface to be treated is generally cleaned so as to remove oil, grease, and other impurities. These impurities may act as mechanical barriers to conversion coating compositions or solutions, and can either interfere with or completely prevent adherence of the conversion coating to the metallic surface.
After cleaning, the metallic surface is typically contacted with a conversion coating solution which usually comprises an acid, a phosphate source, an oxidizer and an accelerator. The surface is then generally rinsed with water to remove unreacted reagents and phosphate salts. Finally, a chromate, nitrate, or acid sealing rinse may be applied to the surface being treated, prior to painting.
Several disadvantages inherently plague conventional conversion coating methods, such as iron phosphate coating methods. Key among these is that iron phosphate processes generally produce coatings which provide less corrosion resistance than zinc phosphate coating processes. Since zinc phosphate processes are generally more complex and more costly to utilize, and are environmentally undesirable, there is a long-felt need in the art for a convenient, inexpensive method of providing corrosion-resistent conversion coatings. This need is met by the instant method and compositions.
It is therefore an object of this invention to provide compositions and a method for applying a uniform, durable coating to a metallic surface which provides corrosion resistance to the substrate being treated. This object is accomplished by utilizing coating compositions which contain tellurium and/or selenium ion sources to form a coating characterized by the presence of tellurium and/or selenium. Any metallic surface can be treated according to the instant invention, including but not limited to galvanized surfaces, stainless steel surfaces, mild steel surfaces and aluminum surfaces. The instant invention is especially effective relative to mild steel surfaces.
This and other objects of the instant invention are accomplished by the novel compositions and method disclosed herein. The instant coating compositions and method allow the application of uniform conversion coatings to metallic surfaces. The method can be utilized at any temperature up to boiling, and the resulting coating provides corrosion resistance to the substrate. The instant coatings also improve the appearance of paints and other coatings subsequently applied to treated metallic surfaces.
U.S. Pat. No. 3,647,568 discloses the use of copper salts in iron-containing phosphate compositions used to produce a red coating on carbon steel. The presence of iron causes the treated surface to be more corrosive if the process of this patent is used.
The MERCK INDEX, Tenth Edition, discloses that tellurium is a reagent which produces a black finish on silverware.
U.S. Pat. No. 4,713,121 discloses phosphate conversion coatings which contain first and second divalent metal elements, such as cobalt and zinc.
U.S. Pat. No. 4,391,855 discloses a coating method which utilizes compositions containing a powdered metal dispersed in a bonding material as a corrosion inhibitor.
U.S. Pat. No. 4,149,909 discloses the use of chlorates and bromates as accelerators and hydroxylamine sulfate as a reducing agent in phosphatizing compositions used to produce iron phosphate coatings.
U.S. Pat. No. 4,595,424 discloses phosphate coating solutions for use on zinc surfaces which contain a phosphate ion source, a zinc and/or manganese ion source and a complex of fluoride ions.
U.S. Pat. No. 4,634,295 discloses a method for improving corrosion resistance of metal substrates which requires application of a direct current to a previously zinc-phosphated metal surface in an acidic solution containing zinc, phosphate and chloride ions.
These references do not disclose or suggest the instant invention.